There are many differences to enumerate between the US and other countries who seem to have fewer mass killings. The knee-jerk seems to be to list guns as the only meaningful difference, and while it certainly plays some role (it's easier to kill people with guns than a knife), I fear the issue is deeper.

Personally, I suspect it has something to do with our culture of self-reliance and personal responsibility, which tends to orient people away from forming a strong community and rely instead on markets and competition. While I'm very pro-market and pro-competition, I suspect people who can't compete in the market end up coping poorly and grow to resent our society. (A friend of mine hypothesized this might be why so many people are questioning their gender, and why gender transitions are 2-4x as likely to be male to female.)

I also think it's no coincidence that shootings are up 50% in the last year while the poorer have only gotten poorer.